HOUSTON -- Nationals left-hander Scott Olsen continued to throw on the side Tuesday in his recovery from an injury to his left shoulder.

Olsen, who is 2-2 with a 3.77 ERA in eight starts, was placed on the disabled list May 22.

"I've been throwing every day since Saturday," Olsen said. "It's getting there.

Olsen said there is no timetable for his return. His throwing has been limited to long-tosses, which reached about 150 feet Tuesday.

Olsen said he had been doing some new exercises to help strengthen his arm the past week.

"So far it's helped," Olsen said of the new workouts. "I've got more range of motion. I can get through my pitches playing catch a lot better so far. Hopefully it will carry over to the mound, which we should be able to do in the next couple of days."

He said a Minor League rehab assignment is a possibility.

"I might," he said. "We haven't gotten that far yet. We haven't had that discussion. It's certainly a possibility."

The arrival of top prospect Stephen Strasburg will not figure into Olsen's return.

"It's not going to affect my decision," he said. "We all knew he was coming. We've all known since Spring Training he was coming."

Getting on base no problem for Willingham

HOUSTON -- Nationals outfielder Josh Willingham reached base four times in five plate appearances -- hitting two singles and drawing two walks -- in Monday night's 14-4 win over Houston, putting him in the National League lead for on-base percentage at .438 entering Tuesday, ahead of the Cardinals'Albert Pujols and the Braves' Jason Heyward.

"It's not really an objective of mine to lead the league in on-base percentage," Willingham said. "I don't [watch the stats]."

Willingham entered Tuesday night's game with a solid .282 batting average to go with 10 homers and 35 RBIs. But he is tied for the league lead, with Pujols, in walks at 39.

"I think that's part of my game," Willingham said of his ability to draw walks. "It's something I need to be able to do. It means I've been selective at the plate, which is when I'm at my best offensively."

On-base percentage has become a more important statistic in baseball the last decade.

"It's definitely become more prevalent of a stat the last five years or so," Willingham said. "Basically it's the same thing as a single. I've always been patient at the plate. It comes with learning the strike zone. I think I've gotten better and better at it since I started pro ball."

Nationals manager Jim Riggleman maintained that part of Willingham's success is his composure at the plate.

"He knows the strike zone," Riggleman said. "If the umpire calls something he doesn't think is a strike, he's able to walk away and not lose his composure and not let it affect him the rest of the game. He trusts himself, and the longer he goes, the calls are going to go his way. He'll make the pitchers throw him good pitches to hit."

Nats uncertain who will go for Strasburg

HOUSTON -- The arrival of Stephen Strasburg will obviously cause some changes in the Nationals' rotation.

Strasburg is scheduled to make his Major League debut next Tuesday against the Pirates. But who will be bumped out of the rotation to make room for the hot right-handed prospect?

"We've been thinking about that for a while," manager Jim Riggleman said. "It's certainly nothing we're going to announce, partially because we don't know yet, but we can whittle it down to two or three different guys. It's going to be a tough call."

Luis Atilano improved his chances of staying in the rotation by outpitching Tim Lincecum and Roy Oswalt in his past two starts. Monday's win over Houston improved his record to 5-1.

Livan Hernandez, with a 4-3 record and sparkling 2.15 ERA, should also be safe.

John Lannan, Craig Stammen and new arrival J.D. Martin would appear to be the most vulnerable. Left-hander Scott Olsen is on the disabled list.

"We've had a pretty idea for a while [when he would join us]," Riggleman said. "We never totally nailed it down. We knew it would be between the first and 15th of June. Things happen. Things can make a change. It's pretty well set, but it's tentative at the same time. There's other things to take into consideration, [Minor League] options, moving a guy to the bullpen as opposed to sending him down. There's a lot of factors in the decision."

Guzman remains in leadoff spot for Nats

HOUSTON -- Nationals manager Jim Riggleman batted Cristian Guzman leadoff for the second consecutive game Tuesday, with Nyjer Morgan hitting second in the order.

Morgan had batted first for much of the season but struggled through the month of May.

The switch certainly worked well in Monday's game, as the pair combined for six hits, a walk and a sacrifice in the Nationals' 14-4 victory over the Astros.

"We're trying to get Nyjer more relaxed," Riggleman said. "We'll keep him there for a little while and see how quickly we can get him back into the leadoff spot.

"I think he'll be more comfortable there [batting No. 2] and get his game back to where he wants it. It's crucial that he's playing good baseball for us."

Guzman, the National League's leading hitter with a .327 average entering Tuesday, didn't care where he hits.

"It doesn't matter for me," said Guzman. "It worked [Monday], 14 runs."

"If Guzzy has to hit leadoff for a while, then that's the way it will be." Riggleman said. "He's not your prototypical leadoff hitter. But Nyjer's on-base percentage [.327] is lower than Guzman's [.353] at this time. You saw the Cardinals make a switch with Albert Pujols going from three to four [in the batting order], and now they've switched him back."